30 years ago BSDs already had non-blocking /dev/random (there was no difference to /dev/urandom). OpenBSD especially wouldn’t have shipped something known insecure. Blocking random probably caused more issues (DOS, random hangs, etc.) than a no blocking CSPRNG would have.
Linux did /dev/random first, so naturally it had the oldest design for a few years, without the security expert scrutiny and experience, which the other OSes had for their implementations.
OpenBSD didn't exist yet when /dev/random and /dev/urandom were created for Linux.